Bacterial meningitis causes approximately 1,70,000 annual deaths, with at least 5-10% case fatality in industrialized countries and a 20% case fatality in the developing world. Streptococcus pneumoniae, Haemophilus influenzae type b (Hib) and Neisseria meningitidis are responsible for most of the cases of bacterial meningitis worldwide.
In total 13 different serogroups namely A, B, C, D, 29E, H, I, K, L, W135, X, Y and Z of N. meningitidis have so far been identified, but about 90% of the infections are due to serogroups A, B, C, Y and W135. Whereas, serogroup X of N. meningitidis (Men-X) recently emerged as a substantial threat to public health. The occurrence of serogroup X was reported in North America, Europe, Australia, and West Africa.
Vaccination is considered to be the most effective way for controlling the spread of infectious diseases. There are several meningococcal vaccines which cover meningococcal serogroups A, C, Y and W, however, currently there is no licensed vaccine in the market which can protect from meningitis caused by serogroup X. So, there is a need to develop more comprehensive vaccines capable to offer broader protection covering serogroup X.
Currently conjugate vaccines are developed to offer higher protection against polysaccharide antigens which is created by covalently attaching a poor (polysaccharide) antigen to a carrier protein, preferably from the same microorganism), thereby conferring the immunological attributes of the carrier on the attached antigen through a T-cell dependent immune response.
Advances in the synthesis of oligosaccharides or polysaccharides, and new technologies developed in biological research have opened a new avenue in carbohydrate vaccine design. Numerous promising carbohydrate-based vaccine candidates have been prepared in recent years which include both naturally occurring carbohydrate and synthetically produced carbohydrate.
The naturally occurring carbohydrates prove to be an important component in the formation of vaccine but they have many drawbacks. The major drawbacks associated with naturally occurring carbohydrate are their isolation and purification which is itself very challenging. Further, any biological contaminants or process impurity which is left behind call for various quality assurance issues. Further, the inconsistency in polysaccharide quality and issues of polysaccharide size distribution lead to batch failures. The bacterial polysaccharides are required to be modified before it can be used for conjugation, leading to damage to the epitopes to varied extent.
While, the synthetic carbohydrate based vaccines have many advantages over the naturally occurring carbohydrates which includes, their well-defined chemical structure. Also there are less chances of any biological contamination and hence offer a better safety profile. Further, synthetic molecules can be modified during synthesis as per requirement to enhance the yield during conjugation and minimizing the damage to immunogenic epitopes during conjugation process by means of an in-built linker attached to the oligosaccharide molecule.
In view of the increasing incidences of the Men-X disease, several methods have been deployed for preparing synthetic Men-X oligosaccharide which can mimic the natural polysaccharide. For instance, the International patent application no. PCT/US2011/037364 titled “Synthetic oligosaccharide for Neisseria meningitidis vaccine” discloses a method for the chemical synthesis of oligosaccharide and conjugate thereof. The invention further provides immunogenic and immunoprotective compositions and antibodies thereof for diagnosing, treating and preventing infections caused by N. meningitidis. Also, a published literature titled “Synthesis of Neisseria meningitidis X capsular polysaccharide fragments” by Laura Morelli and Luigi Lay; Volume 2013, Issue 2, ARKIVOC discloses the synthesis of three conjugatable Men-X capsular polysaccharide fragments.
The existing systems presently in use for the oligosaccharide synthesis involve cumbersome production and purification procedures for synthesizing bacterial Men-X polysaccharides. The methods are either time consuming or give rise to a mixture of different sizes of oligomers. There is a general statement on preparation of successive oligosaccharides, but there is no enabling disclosure on the preparation of Men-X tetramer and further higher MenX oligomers. The above disclosed prior arts teach the chemical synthesis of the Men-X dimer and trimer capsular polysaccharide. They do not disclose the formation of tetramer and are not able to produce high yields of dimer and trimer.
The trimer conjugates reported in the Beilstein J. Org. Chem. 2014, 10, 2367-2376 are not able to provide good immunogenicity. Also the prior art uses −40° C. for O-thexyldimethylsilyl chloride (OTDS) deprotection in the initial steps of synthesis and require 8-9 days for the alpha-phosphorylation, whereas the present invention is carried out at 0° C. to room temperature and is completed in much lesser time. Further, prior art discloses hydrogenation reaction for 24-48 hours whereas present invention requires lesser time for the hydrogenation reaction.